1. Field of the Invention
This invention relates to a colored liquid-crystal display device of the twisted positive dielectric anisotropy nematic type with improved multiplex drive characteristics.
2. Description of the Prior Art
In recent years, interest in twisted positive dielectric anisotropy nematic colored liquid-crystal display devices of the permeable type with color filters and color polarizing plates for use as colored display devices has increased. In particular, to produce liquid-crystal color television with the use of such a display process, research has been active in driving methods, the structure of color filters, liquid-crystal materials, and the like. In this display process, with a liquid-crystal panel with many display picture elements, the two major items that must be achieved are high purity of the color and wide color phase of the color.
As the drive methods for liquid-crystal devices (LCD), there are the pure multiplex drive method and the active matrix drive method; from the standpoints of cost and effective display surface area, the former method is preferable.
Below, the basic principles of a conventional twisted positive dielectric anisotropy nematic-type colored liquid-crystal display device (a color display TN-LCD) with the pure multiplex drive method will be explained with reference to FIG. 7, wherein the reference letter a is the polarizer, b is the analyzer, c is the glass base, d is the molecules of liquid crystals, e is the direction of the axis of polarization, f is the light source, g is the display electrodes (signal electrode and scanning electrode), h is the molecular orientation membrane, i is the common electrode, and j is the color filter layer.
In this way, in an X-Y matrix-type LCD in which there is a combination of a signal electrode (X electrode) and a scanning electrode (Y electrode) that are at right angles to each other, color filter layers j that are red (R), green (G), and blue (B) are formed on the inner side (liquid-crystal side) or the outer side of the glass base c corresponding to the display picture elements, resulting in a twisted positive dielectric anisotropy nematic construction for the liquid-crystal layer, the angle of twist of which is about 90.degree. C. The LCD is used as the light shutter elements. Also, as the polarizing layer (polarizer a and analyzer b), a color polarizing plate with a mosaic-like array of R, G, and B can be used as a full-color display device.
In the drive method, the Y electrode scans along virtual lines, and applies voltage waves that correspond to the selective and non-selective picture elements. As a result, selective picture elements receive effective voltage that is higher than the non-selective picture elements, and at the selective picture elements, the TN-LCD becomes "on" and light of wavelengths corresponding to R, G, and B permeates through. At the non-selective picture elements, when the effective voltage, V.sub.ns, applied to the non-selective picture elements is the threshold value for voltage for the TN-LCD or less, the passage of light through these picture elements is prevented because of the light shutter effect. When the picture elements are small enough compared to the clearly visible distance, the light that passes through the selective picture elements can be recognized as mixed light (full color display). Also, when to the full color display function, the voltage pulse is modulated and an intermediate modulation display function arising from the modulation of the voltage pulse width is added, it is possible to create a color liquid-crystal television.
With the optimized pure-multiplex drive method, the voltage applied to the R, G, and B picture elements cannot be controlled independently, so all the selective picture elements are given the same voltage, V.sub.rms "on". In the same way, all of the non-selective picture elements are given the same voltage, V.sub.rms "off".
When this kind of conventional matrix-type liquid-crystal display device is driven by the optimum voltage mean method with N frequency, the effective voltage V.sub.rms "on" of the selective picture elements and the effective voltage V.sub.rms "off" of the non-selective picture elements have a ratio .alpha. represented by the equation: ##EQU1## In this equation, when N approaches infinity (i.e., N.fwdarw..infin.), .alpha. approaches 1 (i.e., .alpha..fwdarw.1). Therefore, as the frequency N increases, the difference between V.sub.rms "off" and V.sub.rms "on" decreases. Thus, for the manufacture of a liquid-crystal display device of the matrix type with a large capacity for display information, sharp threshold characteristics are needed so that a color display quality that is preferred can be obtained even with a small voltage difference between V.sub.rms "on" and V.sub.rms "off". In particular, in recent years, in the field of liquid-crystal display devices, with the increase in display information, there has been a stronger demand for increased capacity for display information. However, with this kind of conventional TN-LCD, it has not been possible to increase the frequency, N, sufficiently, which causes difficulties in the applications of such a conventional TN-LCD to color liquid-crystal televisions.
A liquid-crystal cell (without a color filter) that has an angle of twist .phi. of the liquid-crystal molecule of greater than 90.degree. has been known as a LCD of the supertwisted birefringence effect (SBE) type (T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 1021 (1984). The feature of SBE-type LCDs is the skillful application of birefringence effects of liquid crystals to increase the threshold characteristics based on the relationship between the applied voltage and the permeation (below, abbreviated "threshold characteristics"), and according to the display colors, there are yellow and blue display modes. FIG. 8 shows the dependence (yellow mode) of the transmittance on the wavelength of the SBE-type LCD in which phenylcyclohexane (PCH) derivatives are used as a positive dielectric anisotropy nematic liquid crystal, the angle of twist .phi. of which is 270.degree., and for which a layer thickness d of the liquid-crystal and a refraction index anisotropy .DELTA.n giving a product .DELTA.n.multidot.d are set at 0.8 .mu.m. It is seen from FIG. 8 that the transmittance changes greatly with changes in wavelength. In this way, the threshold characteristics are much increased, but the transmittance characteristics are highly dependent on the wavelength of incident light because of birefringence effects. In view of these facts, the inventors of this invention investigated the effect of the color display characteristics on .DELTA.n.multidot.d as parameters from the viewpoint of color display, and also investigated the practical limits of these parameters. That is, in order to obtain satisfactory color display characteristics, in addition to making the threshold characteristics suitable, it is necessary that the transmittance characteristics do not depend upon the wavelength. The inventors of this invention therefore studied the use of PCH derivatives for a positive dielectric anisotropy nematic liquid crystal for the conditions where the transmittance characteristics do not depend upon changes in wavelength, and found that in the yellow mode display, when the upper limit of .DELTA.n.multidot.d is set smaller than 0.8 .mu.m, the transmittance characteristics become independent of wavelength, so it is possible to obtain satisfactory color display characteristics. FIG. 3 shows the dependence of the transmittance on the wavelength in the case where .DELTA.n.multidot.d=0.55 .mu.m, indicating that the transmittance characteristics have almost no relation to the wavelength. Moreover, the inventors found that when the lower limit of .DELTA.n.multidot.d becomes extremely small, light cannot be propagated along the direction of twist of the liquid-crystal molecules, and accordingly the function as light shutter elements is damaged. The same results as mentioned above are obtainable even when liquid crystals made of materials other than PCH derivatives are used.